The colours need not stand for anything in specific. All istari were clad in their respective colour upon arrival at Middle-earth already. Olórin, i.e. Gandalf, at any rate used to wear grey even before that in Valinor; or at least always when on 'a journey' (which we know might have already taken him to ME): "Then Manwë asked, where was Olórin ? And Olórin, who was clad in grey, and having just entered from a journey had seated himself at the edge of the council, asked what Manwë would have of him." (UT, istari essay)
The Valinorean names of the istari give no indication of any colour, however.
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